1. Field of the Invention
This invention relates to a method of metallization of a semiconductor device. More particularly, this invention relates to a method for producing a highly reliable contact part, which is adapted for improving conformality of a thin film of a group IVB element, preferably a Ti film employed as a barrier metal, by the electron cyclotron resonance (ECR) plasma CVD method.
2. Description of the Related Art
For filling a metallization material into a contact hole of recent VLSI or ULSI, particularly a contact hole for having substrate-contact with an impurity diffused region in a Si substrate, aluminum (Al) and tungsten (W) are broadly used as an electrically conductive material for filling.
In order to enhance reliability of contact by these filling metals, the inner wall of the contact hole is covered with a barrier metal composed of the group IVB element before filling. A barrier metal which has a two-layer structure consisting of a titanium (Ti) film and a titanium nitride (TiN) film is employed as the above-mentioned barrier metal. The barrier metal having a; two-layer structure is employed for providing a Ti film having a capability for reducing a natural oxidation film on the substrate of the Si substrate to assure ohmic property, and for stacking a TiN film thereon to assure barrier property.
The Ti film and TiN film are formed generally by sputtering. The process for forming the latter TiN film is particularly called "reactive sputtering", in which a Ti target is sputtered in a nitrogen-containing atmosphere.
However, with the sputtering method, the step coverage in a recent contact hole having a high aspect ratio is insufficient. Grains of a film forming material sputtered out from the target are incident on the substrate with a certain directionality. Therefore, the travelling grains are prevented from reaching deep inside of the hole by a shadowing effect from the sidewalls of the contact hole itself.
Thus, the CVD method is expected to be promising as it is capable of forming the barrier metal with a satisfactory coverage on the basis of chemical reactions of the surface in the contact hole.
The TiN film can be formed relatively easily by a known process using various material gases and the CVD method. For instance, an example of forming the TiN film based on methylhydrazine reduction of TiCl.sub.4 using a low pressure CVD device with parallel flat plate single-wafer processing is reported in Monthly Semiconductor World, January 1993, pages 145-151. The formation Gibbs energy in this reaction system of TiN at normal temperatures is approximately -209 kJ/mol (.DELTA.G&lt;0). The system is thermodynamically stable.
On the contrary, the reaction system of film formation of the Ti film by the CVD method is limited to H.sub.2 reduction of TiCl.sub.4 as far as it is known. In addition, the forming Gibbs energy in the reaction system as shown by the following formula is 209 kJ/mol (.DELTA.G&gt;0), which is very high, at temperatures within a range of 100 to 1000.degree. C. for currently practical semiconductor processes. EQU TiCl.sub.4 +2H.sub.2.fwdarw.Ti+4HCl
Therefore, film formation of the Ti film by the conventional CVD method has rarely been realized.
Recently, a technique of forming the Ti film by the ECR plasma CVD method utilizing ECR discharge of high dissociation efficiency of material gases has been proposed instead of the conventional heat CVD method.
However, the Ti film formation by this method is not satisfactory in religibility and reproducibility. For instance, if the Ti film grows into grains, not conformal, under certain conditions, the TiN film growing thereon inherits the surface profile of the Ti film, thus further increasing surface irregularities of the barrier metal. Consequently, problems arise, such as generation of a crack in a corner part on the bottom of the contact hole, and difficulty in filling the contact hole with an upper-layer metallization material in the latter process. The Ti film is a critical component for assuring the ohmic property of the contact. The conformality of the Ti film is a requirement for assuring reliability of the contact part.